A polymer composition or a mixture obtained by polymerizing an ethylenically unsaturated compound in a polyol is referred to generally as a polymer polyol, and is used widely as a raw material for a polyurethane resin such as a polyurethane foam or a polyurethane elastomer. In recent years, a polymer polyol that contains polymer particles having a small average particle diameter has been demanded for the purpose of further improving mechanical properties such as elongation at break. An exemplary polymer polyol containing small-diameter polymer particles is a polymer polyol obtained by a production method in which a ratio of acrylonitrile used as a part of an ethylenically unsaturated compound is increased (see Patent Document 1), which includes a process in which a seed dispersion composed of preliminarily formed submicron particles is prepared by polymerization in the first step, and a monomer is added to the seed dispersion and polymerized in the second step so that a desired polymer concentration is obtained Still further another polymer polyol also is known that is obtained by a production method in which particles are obtained by using preliminarily-formed submicron particles as nuclei (see Patent Document 2).    Patent Document 1: JP 6 (1994)-172462 A    Patent Document 2: JP 9 (1997)-309937 A